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DOE OSTI2024

Reducing crystal symmetry to generate out-of-plane Dzyaloshinskii–Moriya interaction

The Dzyaloshinskii-Moriya antisymmetric exchange interaction (DMI) stabilises topological spin textures with promising future spintronics applications. According to crystal symmetry, the DMI can be categorized as four different types that favour different chiral textures. Unlike the other three extensively-investigated types, out-of-plane DMI, as the last type that favours in-plane chirality, remained missing so far. Here we apply point-group-dependent DMI matrix analysis to show that out-of-plane DMI exists under reduced crystal symmetry. Through strain and structure engineering, we show how C s symmetry is realized in ultrathin magnets and observe the out-of-plane DMI stabilised in-plane chirality using spin-polarized electron microscopy. Our results show that extremely low out-of-plane DMI strengths at µeV/atom are sufficient to stabilise topological spin textures, including merons and bimerons. We also demonstrate field-induced reversible control of the in-plane chirality and merons. Our findings open up untapped paths on topological magnetic textures and their potential applications.

36 MATERIALS SCIENCE↗

DOE OSTI2013

Equation of state of metallic helium

The effective ion-ion interaction, free energy, pressure, and electric resistance of metallic liquid helium have been calculated in wide density and temperature ranges using perturbation theory in the electron-ion interaction potential. In the case of conduction electrons, the exchange interaction has been taken into account in the random-phase approximation and correlations have been taken into account in the local-field approximation. The solid-sphere model has been used for the nuclear subsystem. The diameter of these spheres is the only parameter of this theory. The diameter and density of the system at which the transition of helium from the singly ionized to doubly ionized state occurs have been estimated by analyzing the pair effective interaction between helium atoms. The case of doubly ionized helium atoms has been considered. Terms up to the third order of perturbation theory have been taken into account in the numerical calculations. The contribution of the third-order term is significant in all cases. The electric resistance and its temperature dependence for metallic helium are characteristic of simple divalent metals in the liquid state. The thermodynamic parameters-temperature and pressure densities-are within the ranges characteristic of the central regions of giant planets. This makes it possible to assume the existence of helium in the metallic state within the solar system.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2018

Mictomagnetic State in an EuBaCo{sub 2–x}O{sub 5.5–δ} Single Crystal

The magnetic properties of an EuBaCo{sub 1.9}O{sub 5.36} single crystal are studied in the temperature range T = 2–300 K and the magnetic field range H ≤ 90 kOe. This binary layered cobaltite single crystal has vacancies in the cobalt and oxygen sublattices, in contrast to the stoichiometric EuBaCo{sub 2}O{sub 5.5} composition. All cobalt ions in EuBaCo1.9O5.36 are in a trivalent state. The single crystal has an orthorhombic structure with space group Pmmm, and its unit cell parameters are a = 3.883 Å, b = 7.833 Å, and c = 7.551 Å. The field and temperature dependences of the magnetization of the single crystal demonstrate that it is ferrimagnet below T{sub C} = 242 K. At T < 300 K, all three spin states of the Co{sup 3+} ions are present. The nearest-neighbor interactions give antiferromagnetic (AFM) and ferromagnetic (FM) contributions to the exchange energy. The ratio of the AFM to the FM contributions changes when temperature decreases because of a change in the spin state of the Co{sup 3+} ions. The single crystal exhibits signs of mictomagnetism at low temperatures in high magnetic fields. At T = 2 K and H = 90 kOe, the zero-field and nonzero-field magnetizations are strongly different because of a uniaxial magnetic anisotropy, which tends to set magnetization along the magnetic field applied in cooling throughout the crystal volume. As a result, a complex ferrimagnetic structure with a noncollinear direction of Co{sup 3+} spins appears. The following phenomena characteristic of mictomagnets are also observed in the EuBaCo{sub 1.9}O{sub 5.36} single crystal: a shift in a magnetization hysteresis loop when temperature decreases, retained hysteretic phenomena and no magnetization saturation in high magnetic fields, and an orientation transition. The mictomagnetic state in EuBaCo{sub 1.9}O{sub 5.36} is shown to be caused by the structural distortions induced by vacancies in the cobalt and oxygen sublattices and by the frustration of AFM and FM exchange interactions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2015

Inter-grain interaction in random magnetic anisotropy simulation in magnetic nanocrystals

Effect of inter-grain exchange interaction on the coercive forces was analyzed with a numerical simulation in magnetic materials with random magnetic anisotropy. The magnetization of an assembly of magnetically interacting grains with randomly oriented uniaxial anisotropy was calculated using the Landau-Lifshitz-Gilbert equation. We supposed a single spin model where the magnetizations in a grain were aligned in the same direction, for simplicity. Calculations were carried out for an N×N×N system, where the number of grains on a side, N ranged from 16 to 128. The relation between the coercive forces H{sub C} and the grain size D is represented by H{sub C}∝D{sup k}. With the increase of N, k decreased gradually and tended to reach a saturated value around k = 4.5–5, which dose not correspond to the primitive theory of the random anisotropy model where k = 6. The deviation was discussed in terms of the inter-grain interaction, essentially proportional to the inverse of D.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2015

Spin polarization transfer by the radical pair mechanism

In a three-site representation, we study a spin polarization transfer from radical pair spins to a nearby electron or nuclear spin. The quantum dynamics of the radical pair spins is governed by a constant exchange interaction between the radical pair spins which have different Zeeman frequencies. Radical pair spins can recombine to the singlet ground state or to lower energy triplet states. It is then shown that the coherent dynamics of the radical pair induces spin polarization on the nearby third spin in the presence of a magnetic field. The spin polarization transfer depends on the difference between Zeeman frequencies, the singlet and triplet recombination rates, and on the exchange and dipole-dipole interactions between the different spins. In particular, the sign of the polarization depends on the exchange coupling between radical pair spins and also on the difference between singlet and triplet recombination rate constants.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2014

Magnetic properties of pseudo-ideal cubic La{sub 0.6−x}Ca{sub x}Ba{sub 0.4}CoO{sub 3} with hole doping

The La{sub 0.6−x}Ca{sub x}Ba{sub 0.4}CoO{sub 3} (x=0.03, 0.04, 0.05, 0.075, 0.1 and 0.15) compounds are synthesized. The cubic La{sub 0.6}Ba{sub 0.4}CoO{sub 3} has the strongest ferromagnetic (FM) double exchange interaction among all the La–Ba–Co–O compounds due to the straight Co–O–Co bond. The variation of the lattice parameter versus the calcium content exhibits a non-linear dependence and has the lowest value at x=0.05. In La{sub 0.6−x}Ca{sub x}Ba{sub 0.4}CoO{sub 3} compounds with x≤0.05, the magnetic frustration is observed below about 190 K, and the substitution of Ca{sup 2+} for La{sup 3+} enhances the FM interaction due to the increasing double exchange through Co{sup 3+}–O{sup 2−}–Co{sup 4+} bond. The analysis based on the measured magnetization at 50 kOe and the calculated value indicates that Co{sup 3+} ions are almost equally distributed at the low spin state and the intermediate spin state, and the Co{sup 4+} ions are at the low spin state. La{sub 0.55}Ca{sub 0.05}Ba{sub 0.4}CoO{sub 3} has the lowest coercivity. As the composition deviates from x=0.05, the coercivity is enhanced. For the samples with θ{sub CW}, the substitution of Ca{sup 2+} ions enhances the antiferromagnetic (AFM) interaction, and it is strong enough to make the long range AFM interaction appear. The percentage of the FM domains is calculated, and it decreases very quickly with the Ca{sup 2+} content x≥0.05. The sample with x=0.15 is mostly composed of AFM domains, and its AFM–FM transition temperature is 120 K. Another FM interaction with a low magnetic moment at the low temperature is also observed. - Graphical abstract: The thermal magnetization at 200 Oe at the ZFC and the FC states. - Highlights: • We studied the hole doping on the magnetic properties of La{sub 0.6−x}Ca{sub x}Ba{sub 0.4}CoO{sub 3}. • With x≤0.05, Ca{sup 2+} enhances the ferromagnetic interaction. • With θ{sub CW}, Ca{sup 2+} induces long range antiferromagnetic interaction. x=0.15 is mostly AFM. • Co{sup 3+} ions are equally distributed at LS and IS state, and Co{sup 4+} are at LS state. • We analyzed the development of the magnetic interaction with the doping Ca{sup 2+}.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2015

A study of magnetic ordering in multiferroic hexagonal Ho{sub 1-x}Dy{sub x}MnO{sub 3}

This paper investigates the magnetic properties of Ho{sub 1-x}Dy{sub x}MnO{sub 3} by considering the inter-planar Mn{sup 3+}-O-O-Mn{sup 3+} interaction. The theoretical analysis shows that the asymmetric in-plane exchange interaction couples the in-plane and inter-planar Mn{sup 3+} spin structures via asymmetry parameter δ. This leads to the existence of both the in-plane and inter-planar ordering, which in turn restricted the allowed magnetic space groups to Γ{sub 1} and Γ{sub 4}. The experimental studies confirmed the concomitant nature of the in-plane and the inter-planar ordering at T{sub N}, T{sub SR}, and T{sub 2}. It also showed that the magnetic phase diagram is dominated by the allowed magnetic structures Γ{sub 1} and Γ{sub 4}. Furthermore, an effort is made to resolve the inconsistency regarding the T{sub SR} (32 or 40 K). The studies revealed that the antiferromagnetic inter-planar interaction is switched to the ferromagnetic interaction (40 K) upon cooling, which in turn drives the spin reorientation at 32 K. The Mn{sup 3+} spin structure is seen to be coupled to rare earth sub-lattice through the modulation of the inter-planar interaction.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2019

Magnetic Properties and Mössbauer Investigations of La{sub 0.67}Sr{sub 0.33}Fe{sub x}Mn{sub 1−x}O{sub 3} (x = 0 and 0.5) Perovskites

We report magnetic phase separation in pure La{sub 0.67}Sr{sub 0.33}Fe{sub x}Mn{sub 1−x}O{sub 3} (x = 0 and 0.5) ceramic samples. Rhombohedral crystal structures obtained with R3C space group show no significant changes in the lattice structures due to the close values of ionic radii of Fe{sup 3+} and Mn{sup 3+} ions. However, significant changes occur in the magnetic properties due to competing ferromagnetic (FM) and antiferromagnetic (AFM) interactions because of the presence of Fe{sup 3+} substitution. The FM double exchange interactions (DE) (Mn{sup 3+}–O–Mn{sup 4+}) appears suppressed by AFM superexchange interactions (Fe{sup 3+}–O–Mn{sup 4+}, Fe{sup 3+}–O–Fe{sup 3+}, Mn{sup 4+}–O–Mn{sup 4+}). The saturation magnetization for x = 0 has the highest value of approximately 80 emu/g at 2 K, which dropped significantly to approximately 5 emu/g at 2 K for the x = 0.5 sample. The coercivity (H{sub c}) for x = 0.5 increased from 161 Oe at 300 K to 786 Oe at 2 K compared with that of x = 0 which varied from 110 to 150 Oe. A value of H{sub c}(0) = 1.5 ± 0.04 kOe was extrapolated from the model fit to the temperature dependence of H{sub c}. The temperature-dependent {sup 57}Fe Mössbauer spectroscopy measurements show the presence of Fe in two different environments that have an AFM Fe{sup 3+}–O–Mn{sup 4+} and Fe{sup 3+/2+}–O–Fe{sup 3+/2+} coordination. The unequal fraction population of Fe ions at both AFM environments seems to increase the exchange bias effect.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2024

Finite-Temperature Quantum Matter with Rydberg or Molecule Synthetic Dimensions

Synthetic-dimension platforms offer unique pathways for engineering quantum matter. We compute the phase diagram of a many-body system of ultracold atoms (or polar molecules) with a set of Rydberg states (or rotational states) as a synthetic dimension, where the particles are arranged in real space in optical microtrap arrays and interact via dipole-dipole exchange interaction. Using mean-field theory, we find three ordered phases—two are localized in the synthetic dimension, predicted as zero-temperature ground states, and one is a delocalized phase. We characterize them by identifying the spontaneously broken discrete symmetries of the Hamiltonian. We also compute the phase diagram as a function of temperature and interaction strength for both signs of the interaction. For system sizes with more than six synthetic sites and attractive interactions, we find that the thermal phase transitions can be first or second order, which leads to a tricritical point on the phase boundary. Furthermore, by examining the dependence of the tricritical point and other special points of the phase boundary on the synthetic dimension size, we shed light on the physics for thermodynamically large synthetic dimension.

74 ATOMIC AND MOLECULAR PHYSICS↗

DOE OSTI2023

Temperature dependence of magnetic anisotropy and magnetoelasticity from classical spin-lattice calculations

Here we present a classical molecular-spin dynamics (MSD) methodology that enables accurate computations of the temperature dependence of the magnetocrystalline anisotropy as well as magnetoelastic properties of magnetic materials. The nonmagnetic interactions are accounted for by a spectral neighbor analysis potential (SNAP) machine-learned interatomic potential, whereas the magnetoelastic contributions are accounted for using a combination of an extended Heisenberg Hamiltonian and a Néel pair interaction model, representing both the exchange interaction and spin-orbit-coupling effects, respectively. All magnetoelastic potential components are parameterized using a combination of first-principles and experimental data. Our framework is applied to the α phase of iron. Initial testing of our MSD model is done using a 0 K parametrization of the Néel interaction model. After this, we examine how individual Néel parameters impact the $B$ 1 and $B$ 2 magnetostrictive coefficients using a moment-independent δ sensitivity analysis. The results from this study are then used to initialize a genetic algorithm optimization which explores the Néel parameter phase space and tries to minimize the error in the B 1 and B 2 magnetostrictive coefficients in the range of 0–1200 K. Our results show that while both the 0 K and genetic algorithm optimized parametrization provide good experimental agreement for $B$ 1 and $B$ 2 , only the genetic algorithm optimized results can capture the second peak in the $B$ 1 magnetostrictive coefficient which occurs near approximately 800 K.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2020

Large Dzyaloshinskii-Moriya interaction induced by chemisorbed oxygen on a ferromagnet surface

The Dzyaloshinskii-Moriya interaction (DMI) is an antisymmetric exchange interaction that stabilizes chiral spin textures. It is induced by inversion symmetry breaking in noncentrosymmetric lattices or at interfaces. Recently, interfacial DMI has been found in magnetic layers adjacent to transition metals due to the spin-orbit coupling and at interfaces with graphene due to the Rashba effect. We report direct observation of strong DMI induced by chemisorption of oxygen on a ferromagnetic layer at room temperature. The sign of this DMI and its unexpectedly large magnitude—despite the low atomic number of oxygen—are derived by examining the oxygen coverage–dependent evolution of magnetic chirality. We find that DMI at the oxygen/ferromagnet interface is comparable to those at ferromagnet/transition metal interfaces; it has enabled direct tailoring of skyrmion’s winding number at room temperature via oxygen chemisorption. This result extends the understanding of the DMI, opening up opportunities for the chemisorption-related design of spin-orbitronic devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2014

Spin-phonon interactions of multiferroic Bi{sub 4}Ti{sub 3}O{sub 12}-BiFeO{sub 3} ceramics: Low-temperature Raman scattering and infrared reflectance spectra investigations

Optical phonons of multiferroic Bi{sub 4}Ti{sub 3}O{sub 12}-BiFeO{sub 3} ceramic have been investigated by low temperature Raman scattering and infrared reflectance spectra. Anomalies at about 85 K can be observed from the temperature dependence of the Raman and infrared modes, which arise from spin-phonon interaction during antiferromagnetic to paramagnetic phase transition. It was found that the change of exchange interaction in magnetic phase transition can be induced by Fe-O-Fe octahedral tilting driven from the A-site atoms. Moreover, ferroelectricity-related displacement of Bismuth atoms suggests the coupling of magnetic and ferroelectric orders.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2016

Nuclear symmetry energy in terms of single-nucleon potential and its effect on the proton fraction of β-stable npeμ matter

Momentum and density dependence of single-nucleon potential u{sub τ} (k, ρ, β) is analyzed using a density dependent finite range effective interaction of the Yukawa form. Depending on the choice of the strength parameters of exchange interaction, two different trends of the momentum dependence of nuclear symmetry potential are noticed which lead to two opposite types of neutron and proton effective mass splitting. The 2nd-order and 4th-order symmetry energy of isospin asymmetric nuclear matter are expressed analytically in terms of the single-nucleon potential. Two distinct behavior of the density dependence of 2nd-order and 4th-order symmetry energy are observed depending on neutron and proton effective mass splitting. It is also found that the 4th-order symmetry energy has a significant contribution towards the proton fraction of β-stable npeμ matter at high densities.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2015

Magnetic and electrode properties, structure and phase relations of the layered triangular-lattice tellurate Li{sub 4}NiTeO{sub 6}

We examined the magnetic properties of layered oxide Li{sub 4}NiTeO{sub 6} by magnetic susceptibility, magnetization and ESR measurements and density functional calculations, and characterized phase relations, crystal structure and electrochemical properties of Li{sub 4}NiTeO{sub 6}. The magnetization and ESR data indicate the absence of a long-range magnetic order down to 1.8 K, and the magnetic susceptibility data the presence of dominant antiferromagnetic interactions. These observations are well accounted for by density functional calculations, which show that the spin exchanges of the LiNiTeO{sub 6} layers in Li{sub 4}NiTeO{sub 6} are strongly spin frustrated. The electrochemical charging of Li{sub 4}NiTeO{sub 6} takes place at constant potential of ca. 4.2 V vs. Li/Li{sup +} indicating two-phase process as confirmed by X-rays. The starting phase is only partially recovered on discharge due to side reactions. - Graphical abstract: No long-range magnetic order due to frustration in 2D triangular lattice antiferromagnet Li{sub 4}NiTeO{sub 6}. - Highlights: • Li{sub 4}NiTeO{sub 6} is 2D triangular lattice magnet with no long-range order down to 1.8 K. • Intralayer exchange interactions are antiferromagnetic and strongly spin frustrated. • The electrochemical Li extraction proceeds in a two-phase mode at 4.2 V vs. Li/Li{sup +}. • The electrochemical charge–discharge is only partially reversible. • Li{sub 2}O–NiO{sub y}–TeO{sub x} phase relations are reported; Li{sub 4}NiTeO{sub 6} is essentially stoichiometric.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2019

Influence of a Strong Magnetic Field on Magnetic Order in Polycrystalline Layered Cobaltites EuBaCo{sub 2 –x}O{sub 5.5 – δ}

The magnetic properties of polycrystalline EuBaCo{sub 2 –x}O{sub 5.5 – δ} (x ≤ 0.1, δ ≤ 0.14) are studied in the temperature range 2 K < T < 300 K in magnetic fields H ≤ 90 kOe. A comparison is made of the dependencies of magnetization M on temperature T and magnetic field H for polycrystalline and single crystal EuBaCo{sub 1.90}O{sub 5.36}, in which only Cu(III) ions are present. It is shown that in the magnetically ordered temperature range of magnetic order, the samples are ferrimagnetic, in which antiferromagnetic interactions between the nearest Co{sup 3+} ions with the same spins compete with the ferromagnetic exchange. With a decrease in temperature, a strong magnetic field affects the magnetic order. At T = 2 K in fields H = 50–90 kOe, all samples exhibit the properties of mictomagnetism. There are a shift in the hysteresis loops M(H) and a large difference in the values of the magnetization of the sample cooled in a magnetic field M (FC) and without a field M (ZFC) due to unidirectional magnetic anisotropy. In this case, there is no saturation of magnetization. The field dependences of magnetization taken in the cooling and heating in unequal fields (CHUF) mode indicate a metastable magnetic state. Near T = 20–30 K, an orientational transition characteristic of micromagnets takes place and a complex ferrimagnetic structure is formed. The main reasons for the manifestation of mictomagnetism properties in layered cobaltites are the presence of magnetoactive Co{sup 3+} ions in the states with intermediate and high spins at low temperatures and structural defects that affect the exchange interaction between the nearest neighbors. Nonmagnetic Co{sup 3+} ions in the low-spin state can also act as defects.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2019

Thermodynamic and Magnetic Properties of the Two-Dimensional Anisotropic Ising Model with Competing Interactions

The two-dimensional anisotropic Ising model with competing interactions is studied on a square lattice by Monte Carlo methods using the Wang–Landau algorithm. The temperature dependences of the main thermodynamic and magnetic parameters are calculated. The distinctive features of temperature dependences of these parameters for different values of the competing interaction are shown. The dependences of the wave number of modulated structures on the ratio of exchange interaction parameters are plotted. The phase diagram of the model is built. The coordinates of the Lifshits point and the localization region of the phases with different wave vector values are calculated by interpolating phase boundaries of the d-iagram.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2014

Magnetic and transport properties of cobalt doped La₀.₇Sr₀.₃MnO₃

The magnetic and transport properties of La₀.₇Sr₀.₃Mn{sub 1–x}CoxO₃ (0≤x≤1) samples were systematically studied. A magnetic phase diagram was constructed according to the observed results. At the two ends (x ≤ 0.3 and x>0.9), the samples exhibit the conventional ferromagnetism with the metallic conducting behavior, which can be interpreted in terms of the double exchange coupling between Mn (Co) ions. As for the intermediate Co-doping region (0.3<x≤0.9), the samples enter a mixed magnetic phase state, which displays the characteristics of the glassy states. According to the frequency-dependent results from the ac susceptibility, a representative frequency sensitivity factor K was calculated to be ~0.1 for the typical La₀.₇Sr₀.₃Mn₀.₆Co₀.₄O₃ sample, corresponding to the coexistence of superparamagnetic-like free spins and the reentrant spin glass. The results of the Curie-Weiss fitting of the dc magnetization suggest that the Co³⁺ ions are in the intermediate spin state with the increase of Co-doping level (x ≥ 0.3), while the Co⁴⁺ ions show the spin state transition from the high spin state to the intermediate spin state at x=0.6. The temperature dependence of resistivity shows a metal-insulator transition character for the intermediate Co-doping samples, which can be well fitted by the three dimensional variable-range-hopping model in the whole temperature range. These facts suggest that the double exchange interaction in the perovskite La₀.₇Sr₀.₃MnO₃ is destroyed by the B site doping, due to the strong competition between the double exchange and superexchange interactions in Co/Mn ions as well as the frustration of spins in the system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2014

Thin-film topological insulator-ferromagnet heterostructures for terahertz detection

An atomically thin topological insulator is investigated theoretically for long-wavelength photodetection when it interacts with a magnetic material. Through the coupling between top and bottom surfaces as well as the exchange interaction with the proximate ferromagnet, the distribution of optically excited carriers exhibits unique patterns that depend sensitively on the frequency of the incoming light. This effect results in the generation of strong nonzero photocurrent, leading potentially to room-temperature detection of far-infrared/THz radiation with the advantage of low noise and fast response. The ease of frequency tuning by an external electrical bias offers an added versatility in the realistic implementation.

36 MATERIALS SCIENCE↗